Mix the spices, colouring (if used), lemon juice and garlic and ginger up into a paste with a little water and stir in well with the yogurt to make a tandoori marinade. Slash the flesh of the chicken and marinate in the marinade for 2-60 hours. Preheat the oven to it's highest heat for at least 20 minutes. Shake off excess marinade and place chicken pieces on a wire rack in the oven. Cook for 20 minutes and check the chicken is cooked by piercing the thickest piece with a skewer, if the juices run clear it's cooked, serve with a wedge of lemon.